Bell, 25, had earlier pleaded guilty to charges of failing to provide the puppy with proper and sufficient food and veterinary treatment to alleviate unreasonable and unnecessary pain or distress.
At Christchurch District Court, he narrowly avoided a jail sentence.
Judge David Holderness said the dog was found in a "piteous condition".
After looking at photos of the dog, taken by SPCA inspectors before it was put down, the judge noted: "It looks to be a wretched dog, and it's plain that it's been in that condition for some time."
A necropsy found a total loss of internal fat.
The SPCA said the animal would have suffered undue stress, suffering and pain.
Bell told SPCA inspectors he couldn't afford to take the sickly pooch to a vet and became angry when they came to take the dog away.
Defence counsel Craig Fletcher said Bell had been trying to feed it dog biscuits, but other dogs it was living with were eating them.
When it got very thin, a limited family income meant he couldn't take it to the vet, he said.
"This, frankly, is a serious case of its type," Judge Holderness said.
But "by a narrow margin", he decided to sentence Bell to 14 weeks of community detention, with a 7pm to 6am curfew, and 120 hours of community work.
Bell was ordered to pay $319.90 in reparation to the SPCA, and $1200 toward their legal costs. He has been disqualified from owning any animals for four years.